docs: def extract_mfd(dstore, what):
      """
      Display num_ruptures by magnitude for event based calculations.
      Example: http://127.0.0.1:8800/v1/calc/30/extract/event_based_mfd
      """
      dd = collections.defaultdict(int)
      for rup in dstore['ruptures'].value:
          dd[rup['mag']] += 1
      dt = numpy.dtype([('mag', float), ('freq', int)])
      magfreq = numpy.array(sorted(dd.items(), key=operator.itemgetter(0)), dt)
      return magfreq